1. The Field of the Invention
The present invention relates to vascular access systems and, more specifically, to implantable vascular access ports for use in such systems.
2. Background Art
Implantable vascular access systems are used extensively in the medical field to facilitate the performance of recurrent therapeutic tasks inside the body of a patient.
Such a vascular access system generally includes an implantable vascular access port attached to the proximal end of a vascular catheter. A typical vascular access port has a needle-impenetrable housing that encloses a fluid reservoir that is accessible from the exterior of the access port through a needle-penetrable elastomeric septum. The vascular access port also includes an outlet stem, which projects from the housing and encloses a fluid passageway that communicates with the fluid reservoir. The distal end of the catheter is mechanically coupled to the vascular access port in fluid-tight communication with the fluid reservoir using the outlet stem.
The entirety of the system, both the vascular access port and the catheter attached thereto, is implanted in the body of a patient. The distal tip of the catheter is disposed at a predetermined location where therapeutic activity is to be effected. The distal tip of the catheter is either open-ended or is provided with pressure-sensitive valving that affords for one-way or two-way fluid flow therethrough during use of the system by medical personnel. Once the vascular access system is implanted, the tip of a hypodermic needle can then be employed selectively and repeatedly to access the fluid reservoir of the access port by penetrating the skin at the implantation site for the access port and then by being advanced through the septum of the access port itself.
The syringe associated with the hypodermic needle then is able to deliver medication or other fluids into the fluid reservoir. These flow through the outlet stem of the vascular access port and through the catheter attached thereto, thereby to become infused into the body of the patient at the distal tip of the catheter. Alternatively, the syringe is able to aspirate bodily fluids from the vicinity of the distal tip of the catheter by withdrawing such fluids along the catheter, through the outlet stem and the fluid reservoir of the vascular access port, and lastly up the hypodermic needle into the syringe.
For the repeated selective use of an implanted vascular access port to be successful in the long term, the septum of that vascular access port be possessed of specific properties.
For example, when the tip of a hypodermic needle penetrates the septum, the material of the septum about the shaft of the hypodermic needle must form an effective seal about the exterior of that needle. Otherwise, fluid will escape from the fluid reservoir to the exterior of the vascular access port along the exterior of the shaft of the hypodermic needle. This needle sealing characteristic of the septum of a vascular access port is influenced by several factors, a few of which will be explored subsequently.
The septum should also impose a predetermined amount of needle retention force on the shaft of any hypodermic needle that has penetrated therethrough. Needle retention force refers to the tendency of a septum to resist the removal therefrom of the shaft of any such hypodermic needle. Inadequate needle retention force can allow the tip of the shaft of a hypodermic needle to become withdrawn inadvertently from a septum, even after the tip of the shaft of the hypodermic needle has penetrated the septum to the fluid reservoir in the vascular access port. This is quite painful to the patient and disruptive of the therapeutic process.
If the withdrawal of the hypodermic needle is detected, the attention of medical personnel will, at the very least, need to be redirected to the penetration of the tip of the hypodermic needle through the septum of the vascular access port. If the inadvertent withdrawal of the tip of the shaft of the hypodermic needle from the septum is not detected, however, fluids in the syringe associated with the hypodermic needle will not even enter the fluid reservoir of the vascular access port when infusion of those fluids is undertaken. Instead, the fluids will be injected subcutaneously into the pocket in which the vascular access port is implanted. Necrosis of the tissue surrounding the implantation pocket will occur as a result, complicating therapeutic activities and frequently requiring the removal and reimplantation at another site of the entire vascular access system.
A corollary aspect of the needle retention force imposed on the shaft of a hypodermic needle by any given septum is the degree of force required to cause the tip of that hypodermic needle to advance through the septum from the exterior surface to the interior surface thereof.
This is referred to as the needle penetration force. The needle retention force and the needle penetration force for a given septum are generally identical, but oppositely directed.
It is desirable that the amount of the needle penetration force.be within a range that facilitates the labors of medical personnel. First, the needle penetration force for a given septum cannot be substantial, or the process of accessing the fluid reservoir of the associated vascular access port with the tip of the shaft of a hypodermic needle will be difficult for medical personnel and dangerous to the patient.
On the other hand, the needle penetration force for a given septum should be distinctly different and usually greater than the force required to advance the tip of the shaft of a hypodermic needle through the tissue of the patient at the implantation site for the vascular access port. If such is the case, medical personnel utilizing a hypodermic needle to access the fluid reservoir in a vascular access port will be informed by feel when the tip of the hypodermic needle has actually encountered and is being advanced through that septum. Such tactile feedback has been reported to be particularly useful.
The sealing effectiveness, the needle retention force, and the needle penetration force for a given septum are each in part related to the amount and types of forces applied to the septum by the housing of the vascular access port in which the septum is installed. While torsional forces and tensions are on occasion applied to a septum by the housing of the vascular access port in which the septum is installed, it is more common that the forces applied thereto by a housing are directed inwardly toward the body of the septum. In general, the greater the inwardly directed forces that are applied to a septum, the greater will be the sealing effectiveness of the septum about the shaft of a hypodermic needle. Also, the larger will be the needle retention force and the needle penetration force that are imposed on the shaft of that hypodermic needle by that septum.
The inwardly directed forces imposed on an installed septum by the housing of a vascular access port must, however, not be so great that penetrating the septum with the tip of a hypodermic needle results in coring of the septum. When the tip of a hypodermic needle advances through the septum, coring occurs if any portion of the septum material is forced inside the shaft of the hypodermic needle through the opening in the tip thereof That portion of the septum material forced inside a hypodermic needle in this process is in effect severed from the rest of the body of the septum material.
S Septum coring produces small, detached particles of the septum that are likely to enter the fluid that is infused by the implanted vascular access system into the vascular system of the patient. These particles can obstruct fluid flow through the outlet stem of the vascular access port, or if escaping through the outlet stem of the vascular access port, can become trapped in the cardiovascular system of the patient.
In addition, septum coring produces small passageways through the body of a septum. On occasion these passageways extend entirely through the septum, from the exterior thereof to the fluid reservoir inside the vascular access port. The inwardly directed forces imposed on the installed septum by the housing of a vascular access port should initially urge the material of the body of the septum inwardly upon itself to close such passageways after the shaft of the hypodermic needle is withdrawn therefrom. Nonetheless, continued coring eventually leads to various forms of septum failure that cannot be overcome by such inwardly directed forces. The material continuity of the septum is increasingly compromised, resulting in crumbled areas of the septum matrix. Eventually, leakage of fluid can be expected through the septum from the fluid reservoir in the vascular access port. Once such fluid escapes to the exterior of the vascular access port, necrosis will occur of the tissue surrounding the subcutaneous pocket in which the vascular access port is implanted, causing consequences already described above.
The subcutaneous placement of a vascular access port makes it difficult to predict with precision the location in cross section of the septum of that vascular access port that will be penetrated by a hypodermic needle on any given occasion. The septum installed in the vascular access port should thus exhibit substantially uniform needle sealing, needle retention, and needle penetration characteristics across the entire area of the septum exposed to needle penetration. In this manner, the quality of the interaction between a septum and the shaft of a penetrating hypodermic needle will be substantially independent of the location at which the tip of the hypodermic needle actually enters the septum.
The desirability of producing uniform needle sealing, needle retention, and needle penetration characteristics in a septum has historically mandated that septums be circular in cross section. Uniform stress can be produced in the material of a circular septum by installing the septum in a circular access aperture that has an inner diameter that is smaller than the outer periphery of the septum. The rim of the access aperture then forces the periphery of the septum inwardly in the plane of the septum in a manner that is uniform radially about the entire periphery thereof.
The use of a round septum to produce uniform properties in the installed septum does, however, have drawbacks.
For example, it is desirable that a septum be so installed in the housing of a vascular access port as to present to the exterior of the vascular access port at least a minimum amount of exposed needle target area. This facilitates the locating of the septum by palpation of the skin of the patient at the implantation site of the vascular access port. It also reduces the chances that any given probe by the tip of the shaft of a hypodermic needle through the tissue of the patient at the implantation site will miss the septum entirely.
Missing the needle target area of the septum of vascular access port is a painful event for the patient. It is an event that also presents major risks. If the miss is not detected by medical personnel, the fluids in the associated hypodermic syringe could be injected subcutaneously into the pocket in which the vascular access port is implanted, producing consequences already discussed above.
A large needle target area in the septum of a vascular access port also decreases the likelihood that the desirable repeated selective penetration of the septum by the tip of a hypodermic needle will inadvertently become concentrated over time in any small region of the septum. The dispersal of puncture sites over a large needle target area slows the destructive effects of needle penetration, such as septum coring, and thus contributes to septum longevity.
Circular septums that exhibit a desired minimum amount of needle target area necessitate vascular access ports that are correspondingly large in each direction parallel to the plane of the septum. Vascular access ports of such proportion can only be implanted in correspondingly large tissue areas in the body of a patient, such as in tissue areas in the thigh or in the chest. Occasionally in robust adults, implantation in the upper arm is also a possibility.
The implantation of a vascular access port at these locations is not, however, entirely convenient for repeated ongoing therapy. At these locations, reaching the vascular access port with the tip of a hypodermic needle requires that the patient at least partially undress and remain so undressed during the entire time that the vascular access port is being involved in therapeutic activity. The implantation of vascular access ports in easily accessible portions of the human anatomy, such as in the extremities of an adult patient, would be preferable. There, a vascular access port is easy to locate by palpation and easy to access with the tip of the shaft of a hypodermic needle.
The relatively extensive dimensions of a vascular access port that uses a round septum also precludes the use of such a vascular access port with small children or with infants, as there are simply no large tissue areas in the bodies of such potential patients.
The configuration of a vascular access port to accommodate a round septum also has consequences relative to the manner in which implantation of the vascular access port must occur. Vascular access ports with round septums are correspondingly relatively extensive in each direction parallel to the plane of the septum. As a result, relatively long incisions must be made in the skin of a patient when forming the subcutaneous pocket in which the vascular access port is to be implanted. The longer the incision, the greater will be the duration of the healing process that must occur at the implantation site before therapy can commence using the vascular access port. Correspondingly, greater is the potential for infection or for other complications.